Printed circuit boards are typically populated with many surface-mounted circuit devices. Many small holes are formed in the boards to interconnect the layers of the circuit board. Printed circuit boards are also populated with other types of devices which also need holes formed in the boards.
Drilling machines are typically used to drill the holes in the printed circuit boards. One exemplary type of system is described in U.S. Pat. No. 4,761,876, the entire contents of which are incorporated herein by this reference.
There has been a dramatic increase in the hole count on printed circuit boards, which makes the cost of drilling the holes a significant part of the total production cost. In addition, hole sizes are getting smaller. Small drills are more expensive and can not be fed with the same velocity as larger drills. Due to this fact, drilling time and cost are further increased.
To increase the throughput, higher drill bit rotation rates can be employed. However, there is a limit on the spindle rotation rate, which is due to the effect of the large centrifugal forces acting on the spindle rotors at very high rotation rates. Typically, the spindle is fabricated as a solid rod of steel, which will have a growth in the rotor diameter due to centrifugal force at very high rotation rates. Because the rotor typically is supported on air bearings with relatively small gaps between the bearing structure and the rotor, the growth in the rotor diameter will close or significantly narrow these bearing gaps, leading to seizure of the rotor in the bearings.
Drilling spindles typically use a chuck such as a centrifugal chuck to grip the drilling tool while it is being rotated. Centrifugal chucks are advantageous since the tool can be changed without mechanically operating a release mechanism, as there is no gripping centrifugal force when the chuck is not rotated. Small drill applications can have very small drill bit runout tolerances, which can be difficult to achieve with centrifugal chucks in a single grip configuration.
It would therefore represent an advance in the art to provide a spindle capable of extremely high drilling speeds.
It would also be an advantage to provide a centrifugal chuck capable of gripping tools with significantly reduced run-out.